4 GROWTH IN TREES. 



increase of the volume of the protoplasmfand an^enlargement of the 

 cell-mass of which it forms a part. 



Any instrumentation of sufficient accuracy and delicacy to measure 

 the increases or variations due to growth in the cambium layer will at 

 the same time include the variations in volume of the woody cylinder, 

 which is also the conduit through which liquid passes from the roots 

 to the crown. Similar conditions prevail in nearly all higher plants, 

 and growth measurements of all stems, leaves, and roots include 

 changes in woody or non-living cells. 



Such trees as the birch, with 200,000 leaves, are reputed to transpire 

 as much as 400 liters in a single day. No estimates of the pines are 

 available, but the low moisture content of the soil around the roots of 

 the Monterey pine would not furnish a supply for such use of water. 

 The trunk of a tree may, in fact, be compared to the supply hose of a 

 fire-engine coupled to a hydrant. When the pressure from the mains 

 is enough to supply water faster than it can be pumped out, the hose is 

 distended. When the engine tends to take water faster than it would 

 be delivered by the system, the hose would tend to collapse. Some- 

 thing of this sort takes place in many trees which have been kept under 

 observation. The conduit in this case, however, is not a simple pipe 

 or a set of pipes, but is made up of vessels, through which water may 

 pass under capillary conditions, and inclosed box-like tracheids which 

 may be only partially filled with water. When water is withdrawn 

 from such a system faster than it is taken in, the resulting changes in 

 form and size are complex in character. 



These facts were well considered in my experiments previously 

 carried out when the measurement of growth in trees was taken up 

 in 1918, and a new technique with specially designed instruments 

 was found necessary for the analytical study of the changes in volume 

 of these massive organs. The records are now continuous for a large 

 number of trees for many months, one tree having been under con-- 

 tinuous measurement since September 1919. The chief features 

 considered in the present paper are as follows: 



1. The dendrograph, an instrument for making continuous records 

 of the variation of tree trunks, has been developed to an approximately 

 final form. 1 The essential feature of this instrument consists of a 

 floating frame of metal of low temperature coefficient, such as invar 

 or bario, which may be placed Jaround a. tree trunk, and the variation 



^acDougal, D. T. The dendrograph: a new instrument for recording growth and other varia- 

 tions in the dimensions of trees. Carnegie Inst. Wash. Year Book for 1918, pp. 59-60. 

 . The dendrograph. Carnegie Inst. Wash. Year Book for 1919, pp. 72-78. 



. The course of growth in trees as measured by the dendrograph. Carnegie Inst. 



Wash. Year Book for 1920, pp. 51-52. 



. Measurement of a season's growth of trees by the newly designed dendrometer. 



Carnegie Inst. Wash. Year Book for 1920, p. 52. 

 A brief discussion of the contents of the present paper was given before the American 



Philosophical Society at Philadelphia, April 22, 1921, which is in press in the Proceedings of 



the Society. 



